Method of manufacture of wafers using an electro-chemical residue sensor (ECRS)

ABSTRACT

A method of improving the clean, rinse and dry processes during the manufacture of ICs, MEMS and other micro-devices to conserve solution and energy while completing the process within a specified time. An electro-chemical residue sensor (ECRS) provides in-situ and real-time measurement of residual contamination on a surface or inside void micro features within the sensor representative of conditions on production wafers. The in-situ measurements are used to design and optimize a production process. The wafers are manufactured in accordance with the production process without the ECRS.

CROSS-REFERENCE TO RELATED APPLICATIONS

This applications claims benefit of priority under 35 U.S.C. 120 as adivisional of co-pending U.S. application Ser. No. 11/968,726 entitled“Method Of Design Optimization And Monitoring The Clean/Rinse/DryProcesses Of Patterned Wafers Using An Electro-Chemical Residue Sensor(ECRS),” filed Jan. 3, 2008, which is a continuation-in-part of U.S.application Ser. No. 11/205,635 entitled “Shielded Micro Sensor andMethod for Electrochemically Monitoring Residue in Micro Features”, nowU.S. Pat. No. 7,317,317, Ser. No. 11/205,582 entitled “Micro Sensor forElectrochemically Monitoring Residue in Micro Channels” now U.S. Pat.No. 7,332,902 and Ser. No. 11/205,636 entitled “Surface Micro Sensor andMethod” now U.S. Pat. No. 7,489,141, all filed Aug. 16, 2005, the entirecontents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to monitoring the cleaning, rinsing and dryingprocesses during the manufacture of ICs, MEMS and other micro-devicesand more specifically to optimizing the processes to conserve the clean,rinse and dry solutions and energy using an electro-chemical residuesensor (ECRS) for micro-features including surfaces or void microfeatures.

2. Description of the Related Art

A major challenge in manufacturing of the micro and nano devices is thecleaning and drying of very small void features, particularly those withlarge aspect ratios. These micro features are fabricated in variousprocessing steps and can be very small voids such as gaps, holes, viasor trenches that are intentionally etched. The micro features can alsobe pores in a deposited dielectric material. Cleaning and drying occurrepeatedly during the processing chain and are responsible for asignificant part of the total processing time and for the consumption ofmuch of the water, chemicals and energy.

In semiconductor manufacturing, trenches and vias are fabricated both inthe device level and in the interconnect level. Most of these featureshave high aspect ratios with submicron openings that are orientedperpendicular to the fluid-solid interface of the device to the cleaningfluid and because of their high aspect ratio and very small width arevery difficult to clean and dry. In Integrated Circuits, MEMS and othermicro device manufacturing, well controlled cleaning and drying areessential to avoid deformation of layers and improper adhesion of movingparts. Improper cleaning and drying would have a significant effect onmanufacturing yield and device performance and reliability in bothsemiconductor and MEMS fabrication. Over-cleaning, over-rinsing orover-drying results in excessive use of chemicals, water and energy andalso increases cycle time and potentially causes yield loss. Therefore,there is a strong economic and environmental incentive to use a processthat is “just good enough”.

The fine structures left behind after processes such as etching,deposition, and patterning, need to be cleaned and the reactionby-products need to be removed often down to trace levels. This usuallyinvolves three steps: 1) application of a cleaning solution; 2) rinsingand/or purging using ultra pure water or other rinsing solutions; and 3)drying by removing and purging the traces of any solvents used duringrinsing. Due to the undesirable surface tension associated with aqueouschemicals and non-wetting nature of most future dielectrics, industry ispursing the development of processes based on supercritical fluids suchas supercritical carbon dioxide for cleaning and pattern development.Measurement of cleanliness under these processing conditions is verycritical.

Cleaning, rinsing, and subsequent drying processes are often performedand controlled almost “blindly” and based on trial and error or pastexperience. The way these processes are monitored and controlledpresently is based on ex-situ testing of wafer, chips, or structures.Within the process tool, fixed recipes are provided by tools and processsuppliers. Run-by-run adjustments or control are based on external anddelayed information on product performance or product yields. Thesensors that are currently available are used in the fabs to monitor theconditions of fluid inside the process vessels and tanks, but far awayfrom the inside of micro features (that is what needs to be monitored;it is also the bottleneck of cleaning and drying). The presentmonitoring techniques and devices do not provide realistic and accurateinformation on the cleanliness and condition of micro features.

Industry currently works around this problem while waiting for asolution; the process condition and cleaning and drying are often setwith very large factors of safety (over-cleaning and over-rinsing).Large quantities of water and other chemicals are used (much more thanwhat is really needed). This results in wasted chemicals and water,increased process time, lowered throughput, increased cost, and itcauses reliability issues because of lack of process control.

SUMMARY OF THE INVENTION

The present invention provides a method of improving the clean, rinseand dry processes during the manufacture of ICs, MEMS and othermicro-devices to conserve solution and energy while completing theprocess within a specified time.

This is accomplished using an electro-chemical residue sensor (ECRS)that monitors the remaining contamination while it is being removed frommicro-features including surfaces or void micro features in thepatterned wafers. The ECRS provides for in-situ and real-timemeasurement of residual contamination on a surface or inside void microfeatures within the sensor that represent a surface or micro features onproduction component(s) that need to be cleaned. The ECRS measuresimpedance, and so is very sensitive to the concentration of residualimpurities on the surface and inside the micro feature. The measuredimpedance response of one or more surfaces or micro features within theECRS, namely the magnitude and phase, any sharp transitions or levelingoff provide in-situ information regarding the residual impurityconcentrations and the status of the clean, rinse and dry processes. Thein-situ measurements are used to design and optimize a production runand/or to monitor the production run in real-time to control the processconditions and transfer of a patterned wafer through the processes.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the removal of a surface layer of unwantedresidue from a vertical micro feature showing the concentration profileof the removed residue inside the micro feature;

FIGS. 2 a through 2 d are cross section views of the surface at variousstages during removal of a surface layer of unwanted residue showing thespecific physical processes that affect the contamination removal rate;

FIGS. 3 a-3 c are diagrams illustrating the specific physical processesfor the dry process;

FIG. 4 is a plot of impedance vs. time for a representativeclean/rinse/dry cycle;

FIG. 5 is a flowchart for using an ECRS to design an optimizedclean/rinse/dry process;

FIGS. 6 a and 6 b are impedance plots illustrating the dependence of theclean process on process conditions for cleaning chemical concentrationand temperature;

FIGS. 7 a and 7 b are impedance plots illustrating the dependence of therinse process on process conditions for water flow rate and temperature;

FIGS. 8 a and 8 b are diagrams of a conventional rinse process and anECRS-optimized rinse process;

FIGS. 9 a and 9 b are impedance plots illustrating the dependence of thedry process on process conditions for spin rate and temperature;

FIGS. 10 a and 10 b are diagrams of a conventional dry process and anECRS-optimized dry process;

FIGS. 11 a and 11 b are diagrams illustrating the clean/rinse/dryprocesses;

FIG. 12 is a flowchart for using an ECRS to monitor and control aclean/rinse/dry process;

FIGS. 13 a-13 b are section views of an embodiment of an ECRS formonitoring vertical micro-features;

FIGS. 14 a-14 c are section views of an embodiment of a ECRS formonitoring buried micro-channels; and

FIG. 15 is a section view of an embodiment of an ECRS for monitoring amicro-surface.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of optimizing the clean, rinseand dry processes during the manufacture of ICs, MEMS and othermicro-devices to conserve solution e.g. cleaning solution, ultra purerinse water or drying gas, and energy subject to completing each processwithin a time constraint. All high aspect ratio void structures such asvertical micro-features and buried micro-channels and micro-surfacesformed in or on a dielectric will be generally referred to as a ‘microfeature’ hereafter. Void micro features have a void space no greaterthan 10 microns and more typically 3 nm to 3 microns. A surface microfeature is no greater than 100 microns in width and typically 300 nm to30 microns.

An electro-chemical residue sensor (ECRS) monitors either remainingcontamination or contamination being removed from micro features formedin or on a dielectric film on the patterned wafers. The ECRS providesfor in-situ and real-time measurement of residual contamination on orinside micro features within the sensor that represent micro features onproduction component(s) that need to be cleaned. The ECRS serves as a‘proxy’ for the true conditions on or inside the micro-features of theproduction devices on the patterned wafer. The ECRS measures impedance,which is very sensitive to the concentration of residual impurities onthe surface and inside the micro feature. The measured impedanceresponse of one or more micro features within the ECRS, namely themagnitude and phase, any sharp transitions or leveling off providein-situ information regarding the residual impurity concentrations andthe status of the clean, rinse and dry processes. This information canbe used to determine what process variables and specifically how processconditions affect the rate of change of the measured impedance. Thein-situ measurements are used to design and optimize a production runand/or to monitor the production run in real-time to control the processconditions and transfer of a patterned wafer through the processes.

Cleaning, Drying and Rinsing Micro-Features

The cleaning of residue from a micro feature will typically be performedby a sequence of clean/rinse steps followed by a dry step. During aclean, the surface or micro feature is exposed to a cleaning chemical.The cleaning chemical removes residues and particles from the surfacethat may have formed there during prior processing. During the rinse,the surface or micro feature is exposed to ultra pure water. The waterremoves the cleaning chemical. During the dry, the surface or microfeature is exposed to a dry gas, such as dry air, isopropyl alcohol ornitrogen. The gas, sometimes in combination with heat or a spinningmotion removes the remaining moisture.

FIG. 1 illustrates the removal process of a film of residue 10 from avoid micro feature 12 in a dielectric film 14. The residue is slowlyremoved by a cleaning fluid, which in turn becomes contaminated by theresidue. In the bulk 16 of the fluid, away from the surface 18, thefluid flows briskly and there is a very low concentration of residue inthe bulk fluid. Conventional processes monitor the contamination in thebulk of the fluid. Since the transport of the active components of thecleaning fluid into the feature or the transport of the residue out ofthe feature is slow relative to the removal of the residue from thereactor once it is present in the bulk of the fluid, the removal ofresidue will progress from the top of the feature to the bottom.Consequently, the concentration of residue in the cleaning fluid ishighest at the bottom of the micro feature and gets progressively lowercloser to the surface, at which residue exits the micro feature. Theregions 20, 22 and 24 bounded by lines with equal concentration ofresidue 21, 23 and 25 have concentrations of residue that progressivelydecrease as the region is closer to the surface at which the residueexits the micro feature. The removal of the ultimate traces of residuewill occur last at the bottom of the smallest micro features. Thecleaning of these areas are the bottleneck of the process. When thefeature is being rinsed with ultra pure water to remove the cleaningfluid, the cleaning fluid will likewise be rinsed last from the bottomof the micro features. When the feature is being dried to remove therinse water, the moisture will likewise be removed last from the bottomof the micro features. In conventional processes with conventionalmonitoring, sensors measure the bulk properties of the fluid away fromthe surface in which there is a very low concentration of residue.Therefore, the conventional monitoring does not provide an accuraterepresentation of the contamination of the micro features. In thepresent invention, an ECRS 27 includes a pair of electrodes 28 formed indielectric film 14 on opposite sides of void micro feature 12 to measurethe impedance inside the micro feature to provide an accuraterepresentation of the remaining contamination albeit residue, cleaningsolution or water.

The physical mechanisms that are responsible for the progression, andthat limit the rate of progression of the removal of cleaning chemicalduring the rinse of a surface 30 containing multiple micro features areillustrated in FIGS. 2 a-2 d. A vertical micro feature 32, buriedmicrochannel 34 and micro surface 36 are formed in or on dielectric film38. An ECRS 39 is configured with pairs of electrodes 40, 42 and 44formed on opposite sides of the respective micro features to measure theimpedance during the processes in order to monitor the residualconcentration of cleaning chemical inside a vertical micro feature,inside a horizontal micro feature or at the surface as the processprogresses.

Initially, a leaning chemical carryover layer 46 is washed away by therinse water 48 through the process of fluid mixing and motion calledconvection as shown in FIG. 2 a. This process initially does not removea surface layer 50 close to surface 30 where there is no or very littlerinse water motion. When the carryover layer has been removed, thecleaning chemical is released from the surface by desorption into thesurface layer and slowly diffuses away from the surface into the rinsewater flow. Because the concentration of cleaning chemical in surfacelayer 50 is still high as shown in FIG. 2 b, the cleaning chemical canre-adsorb onto surface 30. As the concentration of cleaning chemical onthe surface drops as shown in FIG. 2 c, adsorption ceases to play a roleand the desorption rate from the surface dominates the removal rate.When the surface concentration of cleaning chemical has droppedsufficiently, removal of the cleaning chemical from the micro features32 and 34 becomes rate limiting. The physical processes repeatthemselves inside the micro features as shown in FIG. 2 d.

The physical mechanisms that are responsible for the progression, andthat limit the rate of progression of the removal of rinse water 48during the drying of surface including micro feature 32 are illustratedin FIGS. 3 a-3 c. As shown in FIG. 3 a, initially the predominantmechanism is surface drying from the top surface of the dielectric film.Once the main surface is dry, axial and radial drying inside microfeature 32 will occur as shown in FIG. 3 b. Once the volume of the microfeature is dry, any remaining moisture will be removed via desorptionfrom the side walls of the feature as shown in FIG. 3 c.

For cleaning applications, the presence of a surface deposit will impactthe impedance between the electrodes since it can hinder or help theconduction process or can have a different dielectric constant than thecleaning fluid. For rinsing applications, the presence of ioniccontaminants in ultra pure water changes the resistivity of the watereven if very small concentrations (parts per billion level) are present.Therefore, the impedance measured between two electrodes will dependvery much on the conductivity of the fluid and thus the presence ofions. Even non-ionic impurities, directly and through interactions withother species present, change the dielectric properties and surfaceresponse to ions inside the micro feature, which in turn define theimpedance. Therefore it can be possible to determine during the rinseprocess that the chemical clean was not sufficiently effective inremoving the surface deposits during the cleaning process. For dryingapplications, the removal of the water from the micro feature (replacingit with dry air, pure nitrogen or some other gas) will likewise resultin a measurable change in impedance, since the difference between theconductivity of ultra pure water and gas can easily be detected.Conduction along sidewalls can be measured, so that the amount ofmoisture adsorbed on the sidewalls or (slightly) conducting residualimpurities on the sidewalls will be detected.

An exemplary impedance response 70 for a vertical micro feature isdepicted in FIG. 4. The physical mechanisms responsible for theprogression within and through the clean, rinse and dry processes areevident from the impedance response. The cleaning process removesresidue from the micro feature thereby reducing the impedance along agradient as the concentration of residue is reduced eventually levelingoff at an endpoint impedance value 74 representative of a clean featureimmersed in the cleaning chemical. The rinse process initially removesthe carryover and surface layers producing a gradual increase inimpedance. Once convection and diffusion become effective to remove thecleaning solution from inside the micro feature, the impedance increasessharply. Once all of the cleaning solution has desorbed from the wallsof the micro feature, the impedance levels off to an endpoint 76representative of a rinsed feature immersed in ultra pure water.Similarly, the dry process initially removes water from the surfaceproducing only a gradual increase in impedance inside the feature. Onceaxial and radial drying become effective to remove moisture from insidethe micro feature, the impedance increases sharply again. Once all ofthe water has desorbed from the walls of the micro feature, theimpedance levels off at an endpoint 78 representative of a dry feature.

For a given production process (specified process conditions for anumber of process variables), the ECRS can be used to monitor theprocesses and determine the endpoint times 74, 76 and 78 for the clean,rinse and dry process, respectively. Alternately, the ECRS could be usedduring a production run to detect the endpoint impedances, terminateeach process and transfer the production wafers to the next process. TheECRS could also be used to detect changes in the impedance to controlcertain process conditions in order, for example, to reduce the amountof solution or energy consumed during a given process.

Production Process Design Optimization Using an ECRS

More interestingly, the rate at which the impedance changes, which is adirect indicator of the rate at which contaminants are removed from themicro feature, may be dependent or independent of the selected processconditions for different process variables. Measurement of the impedancefor a range of process conditions for the different processes andsub-processes can determine whether and to what extent the rate isdependent upon certain process conditions. This information can than beused to determine a low resource solution for each process orsub-process that perform the requisite function within a prescribed timeconstraint. The low resource solution is suitably optimized for somecombination of the amount of solution use and energy expended duringeach process and for the entire process.

An exemplary process for using the ECRS to design and optimize aproduction process for cleaning, rinsing and drying production wafers isillustrated in FIG. 5. The first step is to select an ECRS (step 80) fora particular production wafer. The ECRS may include different microfeatures selected from vertical, buried microchannel and surfacefeatures and may include multiple different geometries for each in orderto provide the impedance measurements necessary to accuratelycharacterize the processes for the production wafers. The ECRS iscalibrated (step 82) for each process subject to design optimization.This is accomplished by measuring the impedance of the ECRS in a knownsolution e.g. the cleaning solution, ultra pure rinse water or dryinggas. The ECRS is then contaminated to imitate the production process(step 84). For example, if design optimization is performed on thecleaning process, the ECRS is contaminated with residue that wouldimitate the last step in the etching and release steps. If designoptimization is skipping the cleaning process, the ECRS may becontaminated with cleaning solution. Design optimization can beperformed on any one, two or all of the processes. In addition, theclean and rinse processes are typically repeated a few times and eachmay be optimized individually.

The next process to be optimized is selected (step 86). The overallprocess and each clean/rinse/dry process have a number of parameters.Some of these parameters such as reactor size and shape are fixed,others such as flow rates, temperature, solution composition and spinrates are variable and some such as a specified time constraint tocomplete the process or specified temperature required at the end of aparticular process may be considered fixed or variable. The processvariables to be investigated and their initial process conditions areselected (steps 88 and 90).

The ECRS is subjected to specified process conditions and the impedanceof the one or more micro features is monitored (step 92) until thecalibrated endpoint impedance is reached (step 94). The ECRS isrecontaminated (step 96), the process conditions are varied over aspecified range (step 98) and steps 92 and 94 are repeated until theendpoint is again reached. For example, the clean process may be testedwith cleaning solutions having different chemical concentrations atdifferent temperatures. Higher concentrations and temperatures generallybeing more effective to clean the devices but requiring more expensivechemicals and energy and generating more waste. The rinse process may betested using different flow rates and temperatures. Similarly higherflow rates and temperatures being generally more effective but also moreresource intensive. Finally, the dry process may be tested usingdifferent spin rates and temperatures. Steps 92, 94, 96 and 98 arerepeated at least once and perhaps multiple times for each processvariable for the investigated process. When complete, if there isanother process (step 100) to investigate control returns to step 86.

Once all the impedance data has been measured for the processes, thedata is used to design the individual processes and perhaps the overallprocess and suitably to optimize the process to conserve resources. Theprocesses are generally not independent of each other. For example, thechemical concentration, immersion temperature and effectiveness of theclean process can impact the rinse process. These interdependenciesshould be considered when designing each process.

For the next selected process (step 102), for each process variable thatwas monitored the dependence of the rate of change of the impedance onprocess conditions is assessed (step 104). This determines whether therate is dependent or independent of process condition, how strongly andwhether that dependency changes during the process. Based on this ratedependency information for the one or more process variables, processconditions to be used in production are selected to provide a lowresource solution (step 106).

The low resource solution is generally subject to the constraint ofperforming the function of the process (i.e. reaching the correspondingendpoint impedance) within a specified time constraint. Alternately, thetime can be treated as a variable for optimization. Other constraintssuch as min/max flow rates, min/max temperatures, min/max processingtimes etc. may also be placed on the solution based on otherconsiderations such as capabilities of the cleaning system, protectionof the wafer or dependencies of other processes. The optimized lowresource solution can be determined by or with the aide of a computerprogrammed to consider the different variables and constraints and thecost of different conditions (solution, energy, time). The steps arerepeated until the last process has been designed (step 108).

At this point, the process designer (or computer) may assess the overallprocess to consider the process dependencies or any other higher levelconsiderations (step 110). These process dependencies could also beconsidered as constraints on the individual processes and incorporatedas each is designed. The result is a production process in which atleast one process has been designed to provide a low resource solution.Thereafter, lots of production wafers are cleaned/rinsed/dried using theoptimized production process step 112).

Clean Process

The performance and rate of the clean process is dictated by the processconditions for the chemical concentration of the cleaning solution andthe temperature of the immersion bath. As shown in FIG. 6 a, the rate ofchange of impedance response 120 increases with increasing chemicalconcentration. As a result, the high concentration solution reaches theendpoint impedance at a time T_(H) 122 well in advance of the specifiedtime constraint T_(C) 124 whereas the medium concentration solutionreaches completion at a time T_(M) 126 near the specified constraint andthe low concentration does not finish the clean process within theprescribed time limit. As shown in FIG. 6 b, the rate of change ofimpedance response 130 increases with increasing temperature. As aresult, the high temperature bath reaches the endpoint impedance at atime T₁ 132 well in advance of the specified time constraint T_(C) 124whereas the medium temperature bath reaches completion at a time T₂ 136near the specified constraint and the low temperature bath does notfinish the clean process within the prescribed time limit.

The dependency of the clean process on the chemical concentration andtemperature as reflected in these plots is assessed and a low resourcesolution that reaches the endpoint impedance within and preferably neartime constraint T_(C) is selected. For example, a medium concentrationsolution at medium temperature conserves chemicals and energy whilesatisfying the constraints. It is important to note that without theprecise impedance measurements over a range of process conditions todetermine these dependencies a conventional clean would typically usehigh concentration chemicals at high temperatures to ensure an effectiveclean in the prescribed time, thus wasting resources.

Rinse Process

The performance and rate of the rinse process is dictated by the processconditions for the flow rate of the ultra pure water rinse solution andthe temperature of the solution. As will be illustrated, the ratedependency on each of these process conditions changes during the rinseprocess creating additional opportunities for resource optimization.

As shown in FIG. 7 a, the rate of change of impedance response 140increases with increasing flow rate during an initial purge regime 142but is largely independent of flow rate during a subsequent desorptionregime 144. The different regimes are demarcated by a breakpointimpedance 146 below which the clean up rate is dependent on flow rateand above which the rate is not dependent on flow rate.

As a result, the high flow rate rinse reaches the breakpoint impedanceat a time T_(HB) before the medium and low flow rate rinses at timesT_(MB) and T_(LB), respectively. It follows that the high flow raterinse also reaches the endpoint impedance 148 at a time T_(HE) beforethe medium and low flow rate rinses at times T_(ME) and T_(LE),respectively. However, because the desorption rate is independent offlow rate the overall time difference is dictated solely by the rate inthe purge regime. A low resource solution may use a high flow rateduring the purge regime and transition to a low flow rate during thedesorption regime.

As shown in FIG. 7B, the rate of change of impedance response 150 isindependent of rinse temperature during an initial flush of the tank butincreases with temperature during a subsequent desorption regimedemarcated by breakpoint impedance 152. The demarcation roughlycorresponds to the purge and desorption regime for flow rate; during thepurge the rinse is temperature independent but not so during desorption.As a result, the hot rinse reaches the endpoint impedance at a timeT_(HE) before the cold rinse at T_(CE). Note the endpoint impedancevaries with the temperature of the rinse, which must be consideredduring calibration. A low resource solution may use a cold rinseinitially and transition to a low flow rate during the desorptionregime.

The dependency of the rinse process on the flow rate and temperature asreflected in these plots is assessed and a low resource solution thatreaches the endpoint impedance within and preferably near a timeconstraint is selected. It is important to note that without the preciseimpedance measurements over a range of process conditions to determinethese dependencies a conventional rinse would typically use a high flowrate hot rinse 160 for several minutes followed by a high flow coldrinse 162 (to cool the wafer) and then idle flow 164 to ensure aneffective rinse in the prescribed time, thus wasting resources as shownin FIG. 8 a. By contrast an optimized low resource solution mightinclude a high flow cold rinse 166 during the purge regime, a short highflow hot rinse 168 at the end of the purge regime to heat the rinse tankand then a low flow hot rinse 170 during the desorption regime. A shorthigh flow cold rinse 172 is used to cool the wafers followed by idleflow 174. The total rinse time is unchanged in this example but thetotal quantity of water and the energy required have been reduceddramatically. In this example, approximately 15 gallons of cold waterand 70 gallons of hot water are saved per cycle.

Dry Process

The performance and rate of the dry process is dictated by the processconditions for the spin velocity of the wafer and the temperature andflow velocity of the dry gas. As will be illustrated, the ratedependency on each of these process conditions changes during the rinseprocess creating additional opportunities for resource optimization.

As shown in FIG. 9 a, the rate of change of impedance response 180increases with increasing spin velocity or gas flow rate during aninitial surface drying regime but is largely independent of spinvelocity or gas flow rate during a subsequent desorption regime. Thedifferent regimes are demarcated by a breakpoint impedance 182 belowwhich the clean up rate is dependent on spin velocity or gas flow rateand above which the rate is not dependent on flow rate.

As a result, the high spin velocity rinse reaches the breakpointimpedance at a time T_(HVB) before the low velocity rate rinses at timeT_(LVB). It follows that the high velocity rate rinse also reaches theendpoint impedance 184 at a time T_(HVE) before the low velocity rate attime T_(LVE). However, because the desorption rate is independent ofspin velocity or gas flow rate the overall time difference is dictatedsolely by the rate in the surface drying regime. A low resource solutionmay use a high spin velocity for surface drying and transition to a lowspin velocity during the desorption regime.

As shown in FIG. 9 b, the rate of change of impedance response 190 isnot strongly dependent on temperature during the surface drying regimebut increases with temperature during the subsequent desorption regimedemarcated by a breakpoint impedance 192. As a result, the hightemperature dry reaches the endpoint impedance 194 at a time T_(HTE)before the low temperature at time T_(LTE). A low resource solution mayuse a low temperature gas for surface drying and transition to a hightemperature gas during the desorption regime.

The dependency of the dry process on the spin velocity and temperatureas reflected in these plots is assessed and a low resource solution thatreaches the endpoint impedance within and preferably near a timeconstraint is selected. It is important to note that without the preciseimpedance measurements over a range of process conditions to determinethese dependencies a conventional dry would typically use a moderatespin (6,000 rpm) cold gas 196 for 10 minutes to ensure an effective dryas shown in FIG. 9 a. By contrast an optimized low resource solutionmight include a high spin (10,000 rpm) cold gas 198 for a couple minutesfor surface drying and then a no spin hot rinse 200 for a couple minutesfor desorption cutting the dry cycle time in half.

Production Wafer Processing and Monitoring

To process production wafers in accordance with the low resourcesolution, a number of production wafers 202 are inserted into a cassette204 and processed through a sequence of clean/rinse/dry baths 206 a-206c in a tank 207 as shown in FIG. 11 a. The concentration and temperatureof cleaning solution 208, flow rate and temperature of the rinsesolution 210, and spin velocity and temperature of the dry gas arecontrolled in accordance with the production process. Alternately, asingle production wafer 202 can mounted on a chuck 212 subjected to asequence of clean/rinse/dry sprays 214 as shown in FIG. 11 b. In eithercase, the production processes can be operated with or without an ECRS216. The ECRS can be used to either monitor the impedance of theproduction process to provide feedback to make additional refinements tothe process offline or to provide real-time data to control the processin some manner.

As illustrated in FIG. 12, the ECRS could be used to monitor theimpedance in order to control process conditions or to terminate eachprocess and transfer the production wafer(s) to the next process. Anappropriate ECRS is selected (step 220), calibrated (step 222) andprocessed with the production wafers (step 224) to monitor the impedanceof the micro feature(s) in-situ in real-time (step 226). An operator orcomputer 228 monitors the impedance levels and rates of change and usesthe information to control process conditions (step 230) and/ordetermine when the process endpoint has been reached (step 232). Forexample, the ECRS could be used to control when the rinse transitionsfor cold-to-hot or high-to-low flow or when the dry transitions fromhigh spin velocity to low spin velocity. Once the endpoint is reached,if the current process is not the last process (step 234) the wafers aretransferred to the next process (step 236) and the process continues.

Real-time in-situ monitoring with the ECRS can be used to refine a lowresource optimized solution or can be used with non-optimized solutions.In some situations, a manufacturer may not have the capability or choosenot to optimize the production process. Instead the manufacturer maychoose a conservative process that ensures cleaning but is wasteful ofresources and use the impedance data from the ECRS to control processconditions and transfer of the wafers to save resources where possible.

ECRS Configurations

The ECRS includes one or more micro features that represent microfeatures in micro devices fabricated on the production wafers and meansto measure the impedance of the one or more micro features. This meansincludes at least a pair of electrodes on opposite sides of the microfeature and an impedance analyzer that applies an ac measurement signalbetween the electrodes to measure the impedance of the micro-feature.The means may also include one or more buffers that supply current toone or more conductive guards so that the guard voltage closely tracksthe ac measurement signal voltage to reduce the effects of parasiticcapacitance. As described previously, a ‘micro-feature’ may constitute avertical void structure, a buried micro-channel or a surface.Configurations of exemplary ECRS for each are described below inreference to FIGS. 13 to 15.

ECRS for Vertical Void Micro-features

As shown in FIGS. 13 a and 13 b, an ECRS 260 includes a pair ofelectrodes 262 and 264 in a dielectric 66 on a substrate 68 on eitherside of a vertical void micro feature 270 having an opening 271 thatlies in the plane of the fluid-solid interface 272. As shown, the microfeature is suitably oriented substantially perpendicular to thefluid-solid interface 272 and the dielectric stack. Conductive layers274, 276 and 278, 280 lie above and below electrodes 262 and 264,respectively, on either side of micro feature 270 and the conductors(not shown) that carry the measurement signal 282, which is supplied bythe impedance analyzer 283 via buffer 292 to the electrodes (such as acoaxial cable). A conductive perimeter 284 on either side of electrode262 electrically connects layers 274 and 276 to form a guard 286.Similarly the conductive perimeter 284 on either side of electrode 264electrically connects layers 278 and 280 to form a guard 288. The guardseffectively surround their electrodes except at the edges of the microfeature and at the electrode contacts and electrically shield theelectrodes from the surrounding environment.

The guard 286 is suitably connected to the output 290 of a guard buffer292 that ensures that the guard is always at nearly the same voltage aselectrode 262. Similarly, guard 288 is suitably connected to the output294 of a guard buffer 296 that ensures that the guard is always atnearly the same voltage as electrode 264. The guards are only used toshield the electrodes from the rest of the environment and do nototherwise contribute to the measurement. The current required to makethe guard voltage the same as the electrode voltage is supplied bybuffers 292 and 296, not by the impedance analyzer 283, hence it doesnot distort the measurement signal 282. Each guard buffer has a firstinput connected to opposite sides of the impedance analyzer and a secondinput connected to a buffer output. The buffers have unity gainbandwidth larger than the ac measurement signal frequency to supply therequisite current to the guards.

The AC measurement signal 282 is applied between the two electrodes 262and 264 and the impedance is measured by the impedance analyzer 283 asthe ratio and phase difference between the measurement signal voltageand current. During monitoring, the change in monitor impedance is anindication of chemical removal/addition from the micro feature 270 or ofmotion of a chemical species inside the micro feature. The measureddevice impedance is related to the concentration inside the microfeature or to the surface concentration inside the pores of thedielectric film 266, thereby producing a concentration-versus-timeprofile. With the inclusion of the guard, the effective parasiticcapacitance is sufficiently small to allow an electrical measurement ofthe total impedance between the electrodes to resolve R_(sol'n) and/orC_(dl).

The guards do not necessarily need to completely surround theelectrodes. It may be sufficient for guards to include only conductivelayers that lie above and below the electrodes (no conductiveperimeter). Furthermore, a single guard that lies either above or belowthe electrode may be adequate in some cases.

ECRS for Buried Micro-Channels

As shown in FIGS. 14 a, 14 b and 14 c, an exemplary embodiment of a ECRS330 for monitoring the process of cleaning, rinsing and drying of microfeatures in-situ comprises at least one and suitably several buriedmicro channels 332 in a dielectric layer 334 between dielectric (e.g.,silicon dioxide (SiO₂), silicon nitride (Si₃N₄) and low-K organicmaterials) layers 336 and 338 and oriented substantially parallel to thedielectric stack and sensor's fluid-solid interface 340. The term“buried micro channel” is used to refer to a void micro structure formedbelow and parallel to the fluid-solid interface with at least oneopening through a top dielectric layer to the fluid-solid interface.

At least one and suitably several pair of electrodes 346, 348 (e.g.,Poly-Si, Aluminum or copper) in dielectric layers 336 and 338,respectively, at a fixed separation and spaced a known distance from theat least one opening are exposed to fluid in the micro-channel andconfigured to receive a measurement signal 350 and carry the measurementsignal (voltage and current) to the micro channel. An impedance analyzer352 measures the impedance of the micro channel between the electrodes(ratio of voltage and current and phase difference between the voltageand current). The micro sensor is suitably supported by a substrate 354(e.g. a silicon wafer or a glass slide) having a covering dielectriclayer 356. If the substrate is itself a dielectric the coveringdielectric may be omitted. A capping dielectric layer 358 is formed overthe micro sensor to avoid direct contact between the fluid and theelectrode 346. Each micro channel has at least one opening and suitablytwo openings 342 through the dielectric layers 336, 358 between thechannel 332 and the fluid-solid interface 340 for receiving fluid 344.

As shown, the micro sensor may be configured with multiple microchannels 332 to improve the reliability of the impedance measure. Themicro channels are suitably identical but may have different geometriessuch as length (the maximum distance across the channel opening) anddepth. If the micro channels have different depth, then complexmathematical deconvolution must be performed to determine thecontribution of each channel length to the total impedance. Hence,unless the mathematical form of the dependence of impedance on depth iswell-understood, it is not desirable to include micro channels ofdifferent depth in the same sensor. The micro channels have an aspectratio greater than 1-to-1 (depth-to-width), typically greater than3-to-1 and may exceed 100-to-1. Because the micro channels are formed inthe plane of the dielectric there is really no limit on their depth,hence aspect ratio.

In order to get a more complete characterization of the residue in themicro channel, multiple electrode pairs 346, 348 can be used to measurethe impedance of the micro channel(s) at different distances from theopening 342. The same electrode pair 346, 348 may be used to measure theimpedance of multiple identical micro channels 332 to reduce themeasurement noise by placing the micro channels in parallel.

Although not shown, the ECRS can be provided with a guard and buffercircuit similar to that described for the vertical void micro-feature toreduce the effects of parasitic capacitance on the measured impedance.

ECRS for Micro-Surfaces

The surface of the dielectric can be non-porous, in which case thesurface cleaning process is rate limited by the desorption of speciesfrom the surface or by removal of the species away from the surface. Thedielectric can also be porous, or have other micro features present init. Furthermore, a cell (biologic or other) may be placed on the surfaceand monitored.

As shown in FIG. 15, an exemplary embodiment of a surface ECRS 430 forin-situ monitoring of the process of cleaning, rinsing and drying ofsurfaces and the micro features in those surfaces comprises twoconducting electrodes 432 and 433 (e.g. copper or doped polysilicon witha typical thickness of 1 μm) that lie in the same plane, embedded in thesurface of a supporting dielectric 456 on a substrate 454 (e.g. asilicon wafer or a glass slide), and covered by a thin dielectric layer434 (e.g., silicon dioxide (S_(i)O₂), silicon nitride (Si₃N₄) andnon-porous low-K organic dielectric materials). The covering dielectricmay be as thin as a few nm, e.g. 10 nm or less, and the electrodes maybe spaced as close a few microns, e.g. 2 μm or less. The “active” partof the electrodes lies on the surface of the supporting dielectric. Asurface segment 439 of dielectric 434 is defined between the conductingelectrodes at the fluid-solid interface 440. The electrodes are adaptedto receive an ac measurement signal 450 to measure the impedance ofsurface segment 439 when the micro sensor is immersed in a fluid 436,being rinsed or drying. An impedance analyzer 452 measures the impedance(ratio of voltage and current and phase difference between the voltageand current) of the surface section 439 between the electrodes viaconnectors 442 and 443 (e.g. copper or doped polysilicon) embedded inthe supporting dielectric 456 beneath the electrodes that carry themeasurement signal 450 to electrodes 432 and 433.

Surface segment 439 has an electrical equivalent circuit consisting ofcapacitors 460 and 461 formed between the electrodes and the solutionsurface 438, capacitors 462 and 463 formed between electrodes and thesurface double layers, the surface resistance 464 and the bulk fluidresistance 465. At solid-solution interfaces, an interface double layerforms because charges in the solution that are mobile (ions) respond tothe presence of fixed charges on the solid. The interface double layeris responsible for capacitance C_(dl) (capacitors 462 and 463) betweenthe dielectric 434 and the solution 436, which forms an impedanceZ_(dl)=1/jωC_(dl) where ω is the measurement signal radial frequency inseries with the bulk solution resistance and which shunts the surfaceresistance. The sensor can extract the individual components if theimpedance measurement is performed over a range of measurement signalfrequencies. Non-linear least squares fitting of the impedance data, awell known method from the domain of impedance spectroscopy, results inthe individual component values.

Although not shown, the ECRS can be provided with a guard and buffercircuit similar to that described for the vertical void micro-feature toreduce the effects of parasitic capacitance on the measured impedance.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. A method for the manufacture of wafers, comprising: providing aproduction process for cleaning, rinsing and drying production wafersthat include micro devices having micro features formed on or in adielectric film, said production process providing a low resourcesolution in terms of solution and energy consumed subject to aconstraint that each said cleaning, rinsing and drying process reach aspecified impedance condition of the wafers' micro features within aspecified time constraint, said reaching of the specified impedancecondition based on in-situ impedance measurements of micro features inan electro-chemical residue sensor (ECRS) that are representative of themicro features on the production wafer, said ECRS used only duringdesign of the production process; and processing production wafersthrough the cleaning, rinsing and drying processes controlled inaccordance with the low resource solution of the production process,said production wafers processed without the ECRS.
 2. The method ofclaim 1, wherein the production process specifies a chemicalconcentration and temperature for a cleaning solution based on theimpedance measurements of the ECRS.
 3. The method of claim 1, whereinthe production process specifies a flow rate and temperature for a rinsesolution based on the impedance measurements of the ECRS.
 4. The methodof claim 1, wherein the production process specifies a spin rate andtemperature for a drying gas based on the impedance measurements of theECRS.
 5. The method of claim 1, further comprising: including anotherECRS with the production wafers to monitor the impedance duringproduction.
 6. The method of claim 1, wherein at least one of thecleaning, rinsing and drying processes includes first and secondsub-processes in which the rate of change of the measured impedance issubstantially independent and dependent, respectively, of a processcondition for at least one process variable, said low resource solutionincluding different process conditions for said first and secondsub-processes.
 7. The method of claim 1, wherein the specified impedancecondition comprises a value of an endpoint impedance.
 8. The method ofclaim 1, wherein the specified impedance condition comprises a rate atwhich the impedance changes.